Peripheral snap-fit locking mechanisms and smooth surface finish of tibial trays reduce backside wear in fixed-bearing total knee arthroplasty

Abstract

Background and purpose - Severe backside wear, observed in older generations of total knee replacements (TKRs), led to redesign of locking mechanisms to reduce micromotions between tibial tray and inlay. Since little is known about whether this effectively reduces backside wear in modern designs, we examined backside damage in retrievals of various contemporary fixed-bearing TKRs. Patients and methods - A consecutive series of 102 inlays with a peripheral (Stryker Triathlon, Stryker Scorpio, DePuy PFC Sigma, Aesculap Search Evolution) or dovetail locking mechanism (Zimmer NexGen, Smith and Nephew Genesis II) was examined. Articular and backside surface damage was evaluated using the semiquantitative Hood scale. Inlays were examined using scanning electron microscopy (SEM) to determine backside wear mechanisms. Results - Mean Hood scores for articular (A) and backside (B) surfaces were similar in most implants-Triathlon (A: 46, B: 22), Genesis II (A: 55, B: 24), Scorpio (A: 57, B: 24), PFC (A: 52, B: 20); Search (A: 56, B: 24)-except the NexGen knee (A: 57, B: 60), which had statistically significantly higher backside wear scores. SEM studies showed backside damage caused by abrasion related to micromotion in designs with dovetail locking mechanisms, especially in the unpolished NexGen trays. In implants with peripheral liner locking mechanism, there were no signs of micromotion or abrasion. Instead, "tray transfer" of polyethylene and flattening of machining was observed. Interpretation - Although this retrieval study may not represent well-functioning TKRs, we found that a smooth surface finish and a peripheral locking mechanism reduce backside wear in vivo, but further studies are required to determine whether this actually leads to reduced osteolysis and lower failure rates.

Figure 1.

Locking mechanisms and inlays examined in this study. A. Triathlon knee: peripheral locking mechanism (gray arrows) with tongue and groove (white arrows) lock and a mechanism incorporating retaining wire held by metal barbs (black arrows). Asterisks indicate the anti-rotational central island. B. Genesis II knee with a dovetail locking mechanism (gray arrows) which incorporates a posterior dovetail (white arrows) and a small anterior wall (black arrows). C. Scorpio knee: peripheral locking mechanism (gray arrows) with tongue and groove (white arrows) lock and a mechanism incorporating retaining wire held by metal barbs (black arrows). D. PFC Sigma knee: peripheral locking mechanism (gray arrows) with anterior and posterior tongue and groove locks (white and black arrows). E. Search knee: peripheral locking mechanism (gray arrows) with anterior and posterior tongue and groove locks (white and black arrows). F. NexGen knee: dovetail locking mechanism (gray arrows) with central dovetail (white arrows) and small anterior wall (black arrows).

Figure 2.

The articular and backside of the inlay was divided into sectors for wear evaluation according to the Hood scale.

Figure 3.

Mean Hood scores of articulating side and backside observed in different types of implants. Data were averaged from measurements performed by both observers. Whiskers show SD.

Figure 4.

Analysis of backside wear using optical microscopy (left) and scanning electron microscopy (right). a. Arrows indicate dimpling seen in the Triathlon inlay. b. Triathlon knee, low-magnification SEM. Arrows indicate loss of machining marks. c. Triathlon knee, high-magnification SEM. There are no PE particles visible; arrows indicate partially preserved machining marks. d. Flattening (arrows) of machining marks on the Search inlay. e. Search inlay, low-magnification SEM. Arrows indicate loss of machining marks. f. Search inlay, high-magnification SEM. PE flattening (arrows); no particles visible. g. Genesis II inlay. Arrows indicate flattening of machining marks. h. Genesis II, low-magnification SEM. Flattening of machining marks; arrows indicate craters. i. Genesis II, high-magnification SEM image. Arrows indicate PE debris visible within craters. j. NexGen knee. Unworn PE within screw holes of the tibial tray (arrows) and material abrasion around it. k. NexGen knee, low-magnification SEM. Abrasion grooves and border (arrows) of unworn material. l. multiple wear debris seen within abrasion grooves (indicated by arrows).